Not really understanding how to solve the following problem:

There are 4 card-processing machines in an office. The fastest of these machines processes x cards in 7 hours and the slowest processes x cards in 8 hours. which of the following (7. 2, 7.3, 7.5, 7.6, 7.7) could NOT be the average time per machine for each of the 4 machines to process x cards.

I tried doing the min and max as follows:

min: (3(x/8)+x/7)/4=(29x/56)/4

max: (3(x/7)+x/8)/4=(31x/56)/4

Stuck here.